This invention relates to a fine particle filtration medium including an airlaid composite for use as a filtration medium, which may be particularly useful in the filtration of fine particles from fluids. The medium of the present invention will preferably find utility in liquid filtration applications.
In the liquid filtration industry, there are many types of liquid applications that require filtering. Contaminants found in these liquids vary in size and can range, for example, from sub-micron size to over 300 microns. Because of the wide range of particle sizes found in the industry, there are a variety of media in the marketplace, each of which is specifically designed to filter a particular particle range. The filtration medium of the present invention is particularly useful in filtering fine particles.
Spunbond nonwoven webs have been used in various filtration applications, e.g. coolant filtration, cutting fluid filtration, swimming pool filtration, transmission fluid filtration, room air filtration and automotive air filtration. In liquid filtration applications, especially for large volume filtration applications, e.g., coolant and cutting fluid filtration, contaminated liquid typically is pressure vacuum driven onto a horizontally placed filter medium. Consequently, the filter medium needs to be strong enough to withstand the weight of the liquid, sludge, and the applied driving pressure. As such, liquid filter media need high strength properties in addition to suitable levels of filter efficiency, capacity and durability.
In general, composite filter media are formed by laminating a layer of a microfiber web onto a highly porous supporting layer or between two highly porous supporting layers since the microfiber layer does not have sufficient physical strength to be self-supporting, particularly under use conditions. Consequently, the production process for composite filter media requires not only different layer materials but also requires elaborate layer-forming and laminating steps, making the filter media costly. Although self-supporting single-layer microfiber filter media can be produced in order to avoid the complexity of forming composite filter media by increasing the thickness of the microfiber filter layer, the pressure drop across such thick microfiber filter media is unacceptably high, making the microfiber media unfit for filter applications, especially for high throughput filter applications. Existing microfiber filter media and laminate filter media containing microfiber webs is that they tend to exhibit weak physical properties. Consequently these filter media are not particularly useful for large volume liquid filtration uses.
Other sheet filter media widely used in the industry are cellulosic fiber webs of thermomechanically or chemically processed pulp fibers. Cellulosic fiber media are, for example, commonly used in automotive oil and fuel filters and vacuum cleaner filters. However, cellulosic fiber filter media tend to have limited filter efficiency and do not provide the high strength properties that are required for high pressure, large volume liquid filtration applications.
Yet another group of filter media that have been utilized in liquid filtration applications are calendered spunbonded nonwoven webs, especially polyester spunbond webs. For example, calendered polyester spunbond filter media are commercially available from Reemay, Inc. under the TYPAR trademark. Typically, spunbond filter media are formed by melt-spinning filaments randomly and isotropically depositing the filaments onto a forming surface to form a nonwoven web, and then calendering the nonwoven web to make it stronger, thus forming a sheet filter medium that has a relatively uniform thickness. These calendered sheet filter media exhibit good strength properties. The filter efficiency, however, of these spunbond filter media is, in general, significantly lower than that of microfiber filter media. In addition, the porosity distribution on the surface of the calendered spunbond filter media tends to be non-uniform. This is because when the spun filaments are randomly deposited on the forming surface, the filament density, i.e., the number of filament strands deposited for a given area of surface, of the deposited web, varies from one section to another; and when the deposited fiber web is calendered and compacted to a uniform thickness, the sections of high fiber density and low fiber density form low porosity and high porosity sections, respectively. Consequently, the calendered spunbond filter media tend to have a non-uniform porosity distribution.
There remains a need for economical filter media that provide a highly desirable combination of high filtration efficiency, capacity and high physical strength.
Yet another problem of prior art filters is variability in thickness. Such thickness variation translates into filtration variation as well. Variable thickness also affects convertibility issues. Since these materials are usually fed through machines with nip rollers or belts, wide variation in thickness results in slippage and jams in the machines which decreases production rates resulting in higher costs.
In the development of such products, a filtration medium which provides ample filtering ability, has uniform thickness and which will not break up during handling or use, is needed.
The filter medium including an airlaid composite of the present invention thus exhibits unexpectedly good filtering ability while being presented in a form which is easy to handle and has strength sufficient to avoid the tendency to fall apart.